Morphological Evidence For Evolution
Morphological Differences
- The longest established way to catalogue differences in organisms is to look at their morphological differences
- This is the basis on which all the earliest forms of taxonomic classification were carried out
- For example, animals were classified into vertebrates and invertebrates on the basis of whether they have a distinguishable backbone
Homologous Structures
- Homologous structures (homologies) are body parts that may look and function very differently but share structural similarities
- The limbs of animals are a good example of this; animals have many different mechanisms of motion and limb use, but the basic arrangement of bones in many different types of limbs is very similar
- Eg. The limbs of birds, bats, crocodiles, whales, horses, and monkeys are used very differently and are visually very different, but are structurally very similar to each other
- One explanation for the surprising similarities of these different limbs is that of adaptive radiation; the idea that organisms with homologous structures have all evolved from a shared, common ancestor but have adapted to different environments in the process
- Note that adaptive radiation does not provide proof that these organisms have evolved from a common ancestor, but it is a good explanation for the existence of homologous structures
A homologous structure: the pentadactyl limb
- A pentadactyl limb is any limb that has five digits ie. five fingers or toes
- Pentadactyl limbs are present in many species from many groups of organisms, including mammals, birds, amphibians, and reptiles
- In different species, the pentadactyl limb has a similar bone structure but can enable an animal to move in a very different way
- The human foot evolved for upright walking and running
- Whale flippers enable them to propel themselves through a marine environment
- Bird wings are usually highly adapted for flight
- The limbs of frogs allow them to walk, jump and swim
- Alligator limbs enable them to walk and swim
- Although the individual bones of the pentadactyl limb in these example animals are very different shapes and sizes due to their different mechanisms of locomotion, their layout is almost exactly the same
Pentadactyl Limb Structures Comparison Diagram
The bone structure of the pentadactyl limb of a human, whale, bird, frog, and alligator; they all have the same basic layout despite having evolved for different functions
Vestigial structures
Note that vestigial structures, while different in nature from homologous structures, can also be explained by common ancestry
- Vestigial structures are those that no longer have a function in an organism
- E.g. pelvis bones in snakes and whales and wings in flightless birds
- These structures tend to be homologous to structures that perform a function in other species
- The presence of vestigial structures suggests a shared ancestry with those species that possess a fully functioning equivalent of the same structure
- Vestigial structures are considered to be 'evolutionary leftovers'; they would have had a function in an ancestral organism, but a change in the environment led to loss of use e.g. a group of fish trapped in a dark cave would have no use for eyes
- The presence of vestigial structures does not harm the species in which they are found, so there is no advantage to be gained by losing them completely; hence their persistence